Applying a shot-peening process to a metal part that has been treated by a carburization and quenching process is well known as providing a compressive residual stress near its surface by continuously projecting small spheres, namely, shot.
By observing a cross section of a section near a surface of the metal part that has been treated by the carburization and quenching process, in particular to a gas carburization and quenching process, but before the shot-peening process is applied to it, one can see that the outermost layer is an oxidized one about 5 μm thick, immediately under it is an intergranular oxidized layer 15 μm thick, and under it is a martensitic structure. Both the oxidized layer and the intergranular oxidized layer are below referred to as an abnormal layer on the surface or an imperfect hardening layer. It is believed that they harm the fatigue strength of a product.
Therefore, to improve the fatigue strength, such a shot-peening process is applied to the metal part to ablate the abnormal layer by the collisions of the shot. In a conventional shot-peening process applied to a metal part that has such an abnormal layer on the surface, shot that has a hardness that is greater than that of the surface of the metal part is employed so as to ablate the abnormal layer on the surface of it.
The applicant assessed the Hv hardness of such a metal part from its section. In particular, it was treated by the gas carburization and quenching process, but before the shot-peening process was applied to it. Thus, it found that, regarding the Hv hardness, the oxidized layer as the outermost layer has one of about 300 and the intergranular oxidized layer has one of about 430, although one area of the martensitic structure has one of about 850 or more.
However, there is commercially-available shot that has a Hv hardness of 1000 or more. Thus it is greater than that of the martensitic structure of the metal part.
Therefore, under the condition where the hardness of the shot to be used is greater than that of the surface of the metal part, not only ablating the abnormal layer on the surface, but also even a sound martensitic structure, which is in the state wherein the elements, e.g., Mn and Cr, that improve the hardening, do not move to the grain boundary, can be undesirably ablated. Or, a crack may be generated on the surface of a sound martensitic structure. Thus there is a possibility that the fatigue strength in the metal part will decrease.
Accordingly, it is desirable to provide a shot-peening process that can ablate only the abnormal layer on the surface, without undesirably ablating, or cracking, the martensitic structure, and thus to stabilize, and to further improve the fatigue strength of, the metal part.